The present invention relates to a graphical input device and, more particularly, to a graphical input device that generates an electrical signal representative of coordinates of any particular point on a predetermined input surface at which a character or a diagram is entered by handwriting.
A graphical input device is generally constructed to detect coordinates of a point on an input surface, or writing surface, where information is handwritten and to generate an electrical signal (coordinates signal) indicative of that particular point. This kind of input device has been put to practical use as an input terminal for communications.
A type of graphical input device known in the art includes a pair of sheets which are each provided with a resistive layer and are separated by a predetermined air gap from each other. In this type of device, when information is written by hand on the surface of one of the two sheets, the resistive layers on the sheets are brought into contact at the specific handwritten point so that coordinates representative of that point are detected based on the resulting changes in the resistance across the opposite sides of the respective sheets. For details of such a prior art device, a reference may be made to U.S. Pat. No. 3,959,585 and U.S. Ser. No. 605,651 filed Apr. 30, 1984. While the described type of graphical input terminal has been marketed to serve as an electronic blackboard or like communications terminal, an attempt to develop a graphical input terminal which is provided with lines or rules on its input surface has not been made. This presumably stems from the fact that in such a graphical input device the input surface of which is flexible, lines or rules which may be provided by painting the input surface are apt to come off, due to flection of the input surface and rubbing with a writing implement during entry of information. Meanwhile, a graphical input device having an input surface implemented with a resistive sheet has the problem that errors are introduced in detecting the coordinates due to an uneven distribution of surface resistivity which develops during the course of deposition of a resistive layer. The uneven surface resistivity distribution in turn results from irregular printing of a resistive material and/or an uneven temperature distribution on the resistive surface during solidification which relies on baking. Such errors would cause the graphical information reproduced from detected coordinates to differ in position from the actually entered graphical information.